Injuries to joints are a common medical problem, but due to the complexity of most joints it is often difficult to make accurate diagnoses. This is especially true of the knee. One common injury to a joint is injury to the anterior cruciate ligament (ACL) of the knee. This type of injury, while common, is not always easy to diagnose. In deference to the patient, physicians prefer not to perform surgery on the knee, or indeed on any other joint, in order to exclude a diagnosis. Thus, it is desired to use non-invasive techniques.
One non-invasive way of diagnosing injuries to the ACL is to elicit anterior translocation of the tibia. Anterior translocation of the tibia is an indirect way of diagnosing ACL tears. It is known that, in a healthy knee, the tibia can move (translocate) anteriorly relative to the femur by about 4 mm when an anteriorly-directed force of about 100 N (approximately 25 pounds) is applied to the tibia. Excessive anterior translocation of the tibia under such a force is a direct indication of ligamentous instability in the knee, usually but not always as a result of tears in the ACL. By manually applying an anteriorly-directed force on the tibia, such as by pulling anteriorly on the leg while holding the thigh still, a physician can feel instability in the knee due to ACL injury. (As used herein, the term "leg" refers to that portion of the lower limb below the knee, and the term "thigh" refers to that portion of the limb above the knee.) However, as might be expected, this method is imprecise in that the amount of force manually applied, and the resulting translocation of the tibia, cannot be quantitatively measured. Moreover, uncommonly individuals with healthy knees will exhibit anterior translocation greater than 4 mm, while some injured knees may exhibit only 4 mm or less anterior translocation. This can result in invasive testing on patients without ACL tear. Thus, while the hands of a skilled physician can help make a diagnosis, a quantitatively more precise way of diagnosing ACL injuries, as well as other possible etiologies, is desired.
Magnetic resonance imaging and arthroscopy are two ways of obtaining more accurate diagnoses of joint injuries. Magnetic resonance imaging (MRI) is non-invasive and, although arthroscopy is technically invasive, it is far less insulting to the joint than surgery. MRI and arthroscopic probes thus have enabled physicians to see into a joint without surgery. These techniques have enabled better diagnoses and have reduced unnecessary surgery, but even such techniques have their limits. For example, tears in the anterior cruciate ligament are particularly problematic because it is virtually impossible to get good visualization of the proximal ACL with an arthroscope and sometimes difficult with MRI. The ACL often tears behind the femoral condyles, and it is difficult to insert an arthroscope behind the condyles to view the ACL. Magnetic resonance imaging can "see" behind the condyles, but often lacks sufficient resolution to image the fine detail needed to determine the existence of a tear because MRI is subject to volume averaging artifacts. That is, there is a limit to the resolution available h MRI due to the size of the "voxels," or volume elements, into which the object (in this case the ACL and femoral condyles) being imaged can be divided by the MRI processor. Because of the difficulty in making an accurate diagnosis using arthroscopy and conventional MRI, unnecessary arthroscopy, or even surgery, may be performed, or necessary surgery may be deferred.
MRI examination is also often adversely affected by the physical constraints imposed by the MRI equipment. In order to image a knee, for example, the knee must be placed inside the MRI magnetic coil. The position into which the patient's leg is put can be variable with current technology, and it is sometimes difficult for the patient to keep the leg in the required position during imaging. In addition, in many cases only the patient's thigh is supported, and the weight of the patient's leg and foot levering against the coil housing tend to cause the knee to bend or the tibia to translocate anteriorly. This puts an unknown and unwanted stress on the joint, and consequently the physician can not be certain whether the resulting MRI image shows an injury or merely an effect of an unquantifiable stress on an otherwise healthy knee. On the other hand, it may only variably reveal true ACL injuries, thereby decreasing diagnostic accuracy.
Accordingly, there is a need for a way of diagnosing joint injuries that does not rely solely on manual evaluation, which does not place unwanted and unquantifiable stresses on the joint, and which is not subject to the current limitations on arthroscopic and conventional MRI procedures. There is also a need for a way of diagnosing joint injuries which is compatible with, and complements, conventional MRI procedures. The present invention fills those needs.